Reduction of arcing between electrodes in a cathode ray tube by conducting coating of resistance material on inner wall of tube neck



REDUCTION OF ARCING BETWEEN ELECTRODES IN A CATHODE RAY TUBE BYCONDUCTING COATING OF RESISTANCE MATERIAL ON INNER WALL OF TUBE NECKFiled July 30, 1964 Nov. 28, 1967 J. w. SCHWARTZ ET AL 3,355,617

INVENTOR5 James WSchwari BY BErirandZguier United States PatentREDUCTION OF ARCING BETWEEN ELECTRODES IN A 'CATHQDE RAY TUBE BYCONDUCTING COATING 0F RESISTANCE MATERIAL ON IN- NER WALL OF TUBE NECKJames W. Schwartz, Western Springs, and Bertrand W. Squier, Hiusdale,IlL, assignors, by Inesne assignments, to Motorola, Inc., Franklin Park,Ill., a corporation of Illinois Filed July 30, 1964, Ser. No. 386,228Claims. (Cl. 313-82) ABSTRACT OF THE DISCLOSURE A highly resistantelectrically conductive film comprised of iron and manganese oxide isdisposed on the walls of the neck of a cathode ray tube adjacent thefinal anode of the electron gun assembly and coextensive therewith. Asecond conductive coating having less resistance than the film is joinedto the film and is disposed around the neck of the tube adjacent to andcoextensive with the electrodes of the gun which are at a potentiallower than the final anode. The film is electrically connected to thehigher potential final anode, and the coating is electrically connectedto the lower potential electrodes so that the voltage drop between thefilm and coating and the electron gun assembly is essentially zero.

This invention relates to means for neutralizing tendency to arcingbetween the elements of an electron gun as used, for example, in akinescope. Although the foregoing statement is in terms of oneenvironment in which the principles of the invention may be embodied,the invention may be more broadly recited as the provision of a slightlyconducting layer on or adjacent to the wall of that portion of theenvelope which is generally coextensive with an electrically conductiveelement or assembly thereof, e.g., an electron gun. In the latter casesome member of the group has a lower potential and some other member hasa higher potential, although the principles of the invention areapplicable to the case when only a single element or electrode is at apotential sufficiently high to cause arcing to another conductiveportion of the tube. Depending upon the degree of vacuum, thecomposition and topography of the gun parts and inner surface of thevessel opposite the assembly and the electric potential, arcing mayoccur. Such arcing is generally evidenced as a brilliant white flash inthe affected zone.

In television picture tubes, the recent trend has been to higher voltageat the last accelerating electrode for increased picture brightness, sayas high as 24,500 volts above ground. Furthermore, the trend has been togreater deflection angles for the horizontal scan, say to 120, which inturn, requires smaller neck diameters and therefore reduction in thespacing between the elements of the gun and the interior surface of theneck.

It is well understood that, to minimize arcing, the interior necksurface extending from the terminus of the funnel Aquadag(trademark ofAcheson Colloids Company for their brand of colloidal graphite in water)to the base should be meticulously clean. Mechanical scouring of thissurface by the use of a brush and abrasive powders has been resorted to,as well as an acid wash, both of which have yielded results far fromsatisfactory. If some minute portion of the surface has impurities thenelectron impact is evidenced as a bluish island in that vicinity. Suchimpurities may result during operation of the tube, for example, when anare caused to be formed between burrs, sharp edges or corners of or oncomponents of the gun deposits vaporized metal particles on the glass,

Patented Nov. 28, 1967 ice nothwithstanding steps taken to eliminateburrs and the like sharp discharge lines or points from such components.

Stray conduction, in which electrons impinge upon the neck surfacesadjacent to the electron gun, is particularly conducive to theinitiation of an arc discharge. Unless the secondary emission ratio atthe impinged area is exactly unity its potential will change due to anaccumulation (or depletion) of charge (electrons). At glancing angles ofelectron impact many insulators such as glass tend to exhibit ratiosmarkedly in excess of unity. The impinged point then charges to a higherpotential causing a greater attraction for primary electrons. This leadsto a regenerative situation where sufliciently high fields and strayconduction occurs whereby gaseous ions are generated and arcing acrossthe electrodes commences. The rather large current incident upon thepath of the arc may cause serious damage to the gun parts.

Leakage due to actual or incipient arc discharge is, in the case ofcolor tubes in particular, equivalent to the presence of deflectingplates opposite the gap between G3 and G4, whereby spurious deflectionof a beam or beams may occur and proper convergence of the beams at thescreen be adversely affected. The invention, by minimizing andcontrolling the potential gradient between the gun and envelope, andalong the envelope surface, avoids this problem.

The present invention proposes to eliminate the interior surface of theglass as a potential source of the difliculties aforesaid by interposingan active circuit element between the gun and neck. Such elementcomprises an electrically partially-conductive element preferably in theform of a layer or film applied to the surface of the envelope in apredetermined zone calculated to establish, in cooperation with theelement or elements capable of initiating the arc discharge, a currentpath of high resistivity whereby a controlled surface potential may beestablished. Thus, a potential at some transverse plane of the gun whichwould otherwise serve as a source of stray emission at a lower potentialsees, instead, a substantially equal potential interposed between it andthe envelope. Accordingly, since the space is bounded by essentially thesame potential arcing is precluded. The layer is desirably continuousfrom one end to the other regarded axially of the gun assembly and inthe form of a cylinder or helix. One end of the partially-conductiveelement is connected to the gun at a point of some chosen low potentialand the opposite end to a point of some chosen higher, potential. Statedotherwise, the interposition of the partially-conductive elementprovides a controlled potential gradient along the neck surface andminimizes the field which would otherwise exist between areas of or onthe glass and the gun.

Another object of the invention is to provide the conductive meansreferred to in the preceding paragraph by spraying, painting, orotherwise depositing the same on the interior surface of the neck in anarea generally coextensive axially with that element or element of thegun which, but for the conductive means, represents a possible source ofarcing. The conductive means is characterized in that the same will notrun during application, will, upon drying, become a tenacious, permanentfilm able to accept end terminals and to Withstand such friction as maybe created as the gun is inserted in the neck; it being understood thatat least the forward end of the gun is provided with resilient fingerslocating the same coaxially with the neck.

A further object resides in providing the interposed conducting means asa conductive helix supported on or secured to, the interior surface ofthe glass as contrasted with the painted, sprayed or otherwise appliedlayer just referred to.

An additional object resides in providing a conductive film, preferablycontinuous, over that portion of the interior of the neck whereat thepotentials due to the elements of the gun are likely to induce arcing,and the connection of the forward and rearward edges thereof to high andlow potentials respectively, in order to establish a graduallyincreasing potential across the axial extent of the film correspondingsubstantially with the potential measured along the generallycoextensive length of the gun, in order that only a very small fieldwill subsist between the film and gun in any transverse plane.

Another object resides in providing such film of a composition whichwill yield the desired voltage drop from end to end by the applicationthereof in the form of an exceedingly thin, uniform film applied to theglass.

A further object is to provide a film as aforesaid, together with acontiguous or overlapping edge portion of greater thickness to whichelectrical connection may be more readily made.

Another object is to provide a film as aforesaid characterized by aplurality of thicknesses from one end to the other whereby theconductivity, and therefore the voltage drops therealong may bepredetermined at specific peripheral zones thereof, and in which eachZone may be of different composition for variation in conductivity.

Still another object is to provide a conducting layer for the purposeaforesaid which will not draw current of a value tending to overload thepower supply to which the several electrodes of the gun are connected.

A further object of the invention is to prevent a regenerative buildupof stray emission to the neck walls of the kinescope.

A further object is to provide a surface coating for the neck of thekinescope which exhibits very low electron emission.

A further object resides in providing conductive means to neutralize thetendency to arcing in the neck region of the tube which will permitgreatly increased sparking voltages when the tube is subjected to abreak-down test following assembly thereof.

Another object is to provide conductive means for the purpose mentionedwhich may be incorporated with the neck portion of the tube envelopeprior to its assembly with the funnel portion thereof and which willwithstand the high temperature to which the assembly is subjected inuniting said portions without degradation of the conductive means.

It will be understood that, where herein, reference is made toconductive means" the words are intended to have a relative meaning. Inpractice, the resistance thereof is high in order to obtain, in arelatively short conductor, a relatively high drop in voltagecorresponding essentially with the difference between the highest andlowest voltages at which different electrodes of the gun are operated.

Other advantages of the invention will become evident from the ensuingdescription which, taken with the accompanying drawing discloses apreferred form of embodying the invention in practice.

In this drawing:

FIG. 1 is a perspective view of a typical configuration of cathode raytube as adapted to the reception of television transmission;

FIGS. 2, 3 and 4 are somewhat schematic representations of a neckportion of a television picture tube, as in FIG. 1, enclosing a tripleelectron gun assembly as used in the N.T.S.C. (National TelevisionSystem Committee) color system, the electrodes of two of the guns beingomitted for clarity, although G5 is shown. The differences in the threefigures constitute alternative embodiments of the electricallyconductive means;

FIG. 5 is a somewhat enlarged view of the neck of the tube of FIG. 1,with the base portion thereof broken away to illustrate a furtherembodiment; and

FIG. 6 is a transerse cross-section through the neck I to show part ofan implement capable of use in applying the conductive means of FIG. 5

Broadly regarded, the invention comprehends the pro vision in a cathoderay tube in a zone generally coextensive with the axial extent of theelement or elements capable of initiating arcing, e.g., an electron gun,of conductive means having its ends connected respectively to points ofhigh and low potential of the gun. It will be understood that the words,high and low, are employed in this specification as relative terms andthat the potentials utilized are such as to achieve the principal objectof the invention, namely, neutralization of the tendency of an arc toform between an element or elements of the gun and/or the interiorsurface of the neck. in a preferred form, the conductive means includesa resistance element which is applied to the interior surface as aliquid which, when dried, possesses a sufficiently high electricalresistance and, therefore, substantially provides the desired voltagedrop between its ends. By "desired voltage drop it is intended tocomprehend a drop which is essentially the same as that subsistingbetween the points of highest and lowest potential of the gun, say from25,000 volts to 500 volts at the respective ends. Moreover, by virtue ofthe thus specified drop, the conductive means will be incapable ofpassing current of a value which will cause excessive heating oroverload the power supply. The liquid employed will be characterized byrapidly drying to a permanent film te naciously adherent to the glass,and of a iscosity precluding gumming during application and yet not sofluid as to run whereby to yield a non-uniform layer. In order toestablish the required potentials at the ends of the conductive film,the ends of the element are electrically connected to the correspondingpotential points of the gun by means of straps capable of beingconnected to such ends by pressure contact or by adhesive means whichare also desirably of an electrically-conductive nature. In the case ofa helical film, short-circuiting between adjacent turns of the helicalelement may be avoided by striking a suitable compromise between thevoltage drop per turn and the voltage drop between any given turn andthe neighboring portions of the gun.

("ommercial production of television picture tubes has demonstratedthat, using optimum techniques, potential difference between the gun andneck will not cause arcing. However, the procedures required areexpensive and depend upon utmost skill on the part of the operatives.Obviously this represents an idealized situation not easily realized inpractice. Moreover. the present tendency is towards substantiallyincreased maximum electrode voltage. For example, a tube for colortelevision according to the N.T.S.C. System currently operates with adrop from ultor to cathode (of each gun) of 25,000 volts. (Ultor is anaccepted term referring to the final anode.)

In accordance with the invention, the conductive means may be applied tothe neck as a disconnected component and this latter fused to thefunnel, or the neck may already be part of the tube. In either event,the neck or tube, as the case may be, may be supported to rotate theaxis of the neck whereby to permit uniform application of the film tothe surface of the glass without skipping or variation incross-sectional area.

Thus, referring to FIG. 1, there is shown, by way of example, atelevision receiving tube including a funnel 1t) merging into a neck 11terminating in a base 12 which carries the plurality of terminal prongs(not shown). In accordance with common practice, the interior of thefunnel and a forward portion of the neck is coated with colloidalgraphite (Aquadag) 33 with which the high voltage input terminal (notshown) makes contact. The electron gun is conventionally provided with aplurality of resilient fingers R4 to press against the Aquadag wherebythe high voltage, say 25,000 volts, is applied to the appropriateelectrode 15 GS) of the gun, and G4 (sometimes termed the ultor). Thesefingers 14 also assist in centering the gun on the axis of the neck. Asmentioned and for illustrative purposes, the gun shown in phantom inFIG. 5 is of the three-color type, but it will be understood that thegun could equally be of the monochrome type. Moreover, the terminalsextending from the gun to the base are omitted for clarity. All of thefeatures just alluded to are well understood in this art and have beenrecited in order to set forth a typical environment in which theprinciples of the invention may be carried into practice. It is to berecalled that the gun assembly of FIGS. 2, 3 and 4 is shownschematically and that the same may be as in FIG. 5.

Adverting to FIGS. 2, 3 and 4, the gun electrode 15 will, in theexample, operate at approximately 25,000 volts with respect to thecathode and grid 21 (G1) at approximately 400 volts, or a difference ofapproximately 21,000 volts. Thus, in order to obviate arcing, theinvention contemplates the interposition between the glass neck and theelectrode assembly, or selected portions of the latter, of a conductive,yet highly resistive film or layer. In order to avoid points in the filmwhereby the beneficial effects thereof are adversely affected, it isimportant that the film be free of particles which would otherwise actas emission points, it being remembered that density of the field at apointed electrode varies inversely as the radius of the point.Accordingly, a fluid vehicle containing resistive particles is to beavoided. Additionally, the material of the film should possess thelowest possible rate of secondary emission under bombardment byelectrons emitted from the components of the gun. In addition to causinga regenerative positive charging of the secondary emission arcs,electrons may serve as a conductive connection between the gun and filmand therefore would tend to provide a path for the initiation of arcing.A further desired characteristic of the film is a high degree ofadherence to the glass. Obviously, the silicates, such as sodium orpotassium silicate, would provide suitable vehicles for a highlyresistive particulate substance, and similarly with regard to certainceramic cements. However, these vehicles are known to be readilyemissive in both a primary and secondary sense. It is also necessarythat the film, following de-gassing of the tube, be incapable of furtheremission of gases, in order that the hardness of the tube, followingsealing ofi, remain relatively unaffected. Nor should the film becapable of electrical breakdown at the highest potential to which it maybe subjected.

The conductive film is preferably applied in liquid form by spraying. Tothis end the tube is supported with the axis coaxial with the nozzle ofa spray gun or the like, whereby the liquid is deposited on the glass asa mist. Obviously suitable masks are employed to limit the two endboundaries of the applied film. A suitable mask is a thin, fiat, axiallyslit, resilient cylinder snapped into the neck of the tube with its endsoverlapping. To provide utmost uniformity of thickness of the filmeither the tube or the nozzle is reciprocated axially and/ or bothrelatively rotated.

Experiments have demonstrated that the most suitable substances for theconductive film are of the P-type (as such terminology is used withreference to semi-conductors). In fact, the conductive elementcomprehended by the present invention, being of a highly resistivenature, may also be justifiably referred to as a semi-conductor. In theforegoing connection, it is recognized that P-type materials have verylow rates of primary and secondary emission.

Further referring to FIGS. 2, 3 and 4, the conductive layer is appliedto the inside of the neck 11 continuously around the circumferencethereof, and extends from the Aquadag 13 to a line X which is rearwardlyof the forward end of G3 (Dimension B). The area is crosshatched withlines inclined downwardly from right to left (not only in this figurebut in FIGS. 3 and 4). A

6 coating 26 merges with film 25 at the line X and continues slightlybeyond G1 (Dimension C).

In one preferred embodiment, the layer or film 25 is applied as anaqueous solution having the following formula:

Fe(NO -9H O g 0.58 Mn(NO (51% sol.) ml 28 H 0 ml 200 During baking, thewater is driven ofi and the nitrates are decomposed to yield a tenaciousdurable film which consists esentially of the oxides of iron andmanganese, the resistance per square whereof is on the order of 10 to 10ohms. (Resistance per square is defined in The Encyclopedia ofElectronics, Rehinhold Publishing Corporation, New York, 1962, at page721.) If desired, the glass may be heated during spraying in order toset the film and avoid running. I

Although the preferred formula just specified includes the nitrates ofthe metals, other soluble metallic salts are within contemplation suchas the hydroxides, chlorides and sulfides.

In general, it may be stated that at least one component of the solutionis a good electrical conductor and the other one is relatively poor.Thus by varying the proportions of each, a film may be achieved whichpossesses the optimum resistance per square.

In the example of FIG. 2, the conductive coating 26 (shown bycross-hatching downwardly from left to right) has the formula:

Fe(NO -9HO g 10 H O liter 1 providing a resistance per square of severalthousand ohms. Electrical contact of this coating to a point of lowpotential is effected by means of a resilient finger 29 extending fromG1 and pressing against the coating. Alternatively, the coating 26 maybe Aquadag.

From the foregoing it will have become apparent that the film 25provides a path of high resistance having a decreasing potentialdistribution along its length from the potential of G5 to the potentialof G1, or from say 25,000 volts to 400 volts. Accordingly, consideringany transverse plane through the film 25 the several electrodes see asubstantially equivalent potential at the glass and the potentialdifference which otherwise would have been responsible for arcing hasbeen eliminated.

In the example, and assuming a configuration of elec tron gun per FIG.1, RCA. specifications for the 21CYP22 picture tube (Radio Corporationof America, Harrison, N.J., published May 1957), the dimensions A, B andC are A", Ma" and A3 plus or minus more or less. It will be noted that Crepresents an extension of coating 26 beyond the rear end of G1.

The foregoing embodiment is characterized in that the film 25 iscoextensive axially with those electrodes operating in the range ofhigher potentials, namely, G5, G4 and G3 or from say 25,000 volts to6000 volts, any lower potentials being disregarded as not apt to giverise to arcing. It will be observed that the coating 26 serves to joinfilm 25 to the mechanical terminal 29 and, for that purpose, may besomewhat thicker than the latter and that, for this purpose, the latterwill overlap or be overlapped by, the former.

The embodiment of FIG. 3 may be regarded as analogous to that of FIG. 2except for the interposition of a cylindrical sleeve 31 concentric withthe axis of the neck and there located and retained by a plurality ofresilient spacers 32. In this case the sleeve is put at ground potentialor operated at the most negative potential available at the gun. Thelatter alternative is best realized by means of an electrical connectionbetween the sleeve 31 and G1, e.g., a jumper 33. A sleeve of thischaracter provides electrostatic isolation of the coating 26 where thisis characterized by the inclusion of particles providing highconcentration of electrostatic stress and therefore sparking, forexample, when Aquadag is used. Although grounding of the Aquadag wouldprovide an equivalent result this presents some operational problems.The sleeve is generally coextensive with those components of the gunwhich operate at sufliciently high potentials as to induce sparking atthe "points of the Aquadag. The sleeve should extend forwardly slightlybeyond the junction of films 25 and coating 26 but not so far as toextend into a region where the potential of film 25 is of high value norso little beyond such junction that leakage may cause a significantfield capable of initiating arcing in the region of the coating 26. Inthis embodiment B indicates the rearward displacement of the forwardedge of the sleeve 31 from the forward face of G3 while D indicates theposition of the plane of merger between layers 25 and 26 with respect tothe forward edge of the sleeve. In this example A and C have the samesignificance as before. Specifically, A= /4, B /s", C= /a", D= /s", withtolerances as previously mentioned.

The embodiment of FIG. 4 involves the use of a single resistive coatingon the neck wall. Its purpose is to prevent the potential of wallsections adjacent to low voltage gun parts from rising to excessivelyhigh levels which are capable of causing excessive emission of the gas.and eventual arcing. The regenerative tendency heretofore referred to,is one in which a small amount of tray emis sion from a low voltage gunelectrode strikes a portion of the neck, thereby liberating secondaryelectrons which then travel toward, and are collected by a high voltagegun electrode. If more electrons are liberated than strike the surface,the secondary emission ratio is such as to exceed unity.

In such case, the area impinged upon will become more positive due todepletion of negative charges. This will create a higher voltage in thevicinity and more electrons will be attracted to that area of the neck.More electrons striking mean more electrons leaving, and the creation ofan even more positive potential area on the neck. The potential of thearea so affected may rise by many thousands of volts.

The ever increasing current and striking energy of the stray electronsin this region of the neck may cause the liberation of gas which maythen become ionized whereby to serve as a path for the initiation of anarc discharge Within the neck space. One method of preventing this typeof regenerative (run-away) situation is to control the surface potentialof the neck, as in the previous embodiments.

The embodiment of FIG. 4, however, provides such control over only alimited region, i.e., normally only adjacent to the low voltage gunmembers. Negation of the regenerative condition may be realized bypreventing electrons from striking the neck. In FIG. 4 this isaccomplished by making the neck area adjacent to the low voltageelectrodes at least as negative as that of any electrode. The fieldsbetween this neck region and adjacent gun electrodes will then be suchas to repel the electrons and whereby essentially none will actuallyimpinge on the neck in the coated region.

Since the rearward end of the cathode is apt to emit electrons, thecoating portion 26a is similar to coating 26 in FIGS. 2 and 3 and servesas an efiective suppressor of this and other sources of stray emission.

In the example of FIG. 4, the dimensions are: A /it", B /s" and C /s,with the tolerances similar to those heretofore given.

In any of the foregoing embodiments and those to be referred tohereinafter, it is within contemplation to apply separate films to theconventional glass supports whereby the several electrodes of the gunare supported, on the basis that such supports may function much in thesame manner as the glass constituting the neck of the tube. Theformulation of the resistive film and the application thereof to thesesupports will generally fol- Cir (3 low the principles laid down inconnection with the neck of the tube.

It will be apparent that other formulations for the film 25 may beemployed, for example, soluble salts of iron and titanium in proportionssuch as would yield resistance per square on the order of from 10 to 10ohms. However, it has been found that, in many cases, the change inresistance is extremely sensitive to variations in the mix and thereforeextreme care must be taken to main the predetermined proportions. Thisis particularly the case for the salts of iron and titanium where a 2%change in the proportion of titanium to iron results in a change ofresistance per square by a factor of three or more.

FIGS. 5 and 6 relate to an alternative embodiment where the conductivemeans comprises an element 51 of high resistance in accordance with thebroad concept heretofore set forth. In the present case the element 51is preferably applied to the interior surface of the neck in the form ofa helix having an axial length generally coextensive with the length ofthe gun or some predetermined portion thereof Whereat arcing has beenfound to present a problem. More specifically expressed, the ends of thehelix should, in general, be substantially opposite those portions ofthe electron gun whereat the highest and lowest potentials C.\l\l..Tince G4 is at the highest potential, one end of the helix Will beopposite or slightly beyond. in a forward-looking sense those portionsof the gun which are effectively at the same potential as G4. Since. aswill appear, the pitch of the turns of the helix should be a maximumconsistent with other considerations, a preferred termination of thehelix would be opposite the distal end of the fingers 14 which contactthe Aquadag. If such termination is to be desirably short of theAquadag, then a strap or similar device may be employed between theforward end of the helix and G4, or a part electrically joined thereto.By way of example, FIG. 5 shows the forward termination of the helixmerging with the Aquadag at 52.

The opposite end of the element 51 is positioned opposite some suitablepoint of low potential which, but for the element 51, could give rise toarcing. While a point of lowest potential of the gun could be utilized,such connection would give rise to a greater voltage drop over thelength of the element 51 and corresponding difficulty because of thegreater drop between turns and danger of arcing or breakdown as to theelement per se. Consequently, it is preferred to connect the reartermination of the element to G3 which, in the RCA. gun heretoforereferred to, operates at a potential of 4,500 volts. The connection maybe made by means of a strap 54 secured to G3 and pressing against orunited with, the termination of the helix. Such latter termination maybe connected in any suitable fashion, either by rendering the strapresilient to bear on the termination or by means of an adherentsubstance, e.g., Aquadag.

A preferred mode of applying the element 51 is by painting a helix ofappropriate cross-section, coefiicient of resistivity and effectivelength as will yield a safe value of current and substantially therequired voltage drop. Any known electrically-conductive fiuid havingthe required characteristics will serve, provided that the same retainsa substantially uniform viscosity for flow from the painting implementonto the glass, will withstand the friction of the forward resilientfingers of the gun as this latter is inserted in the neck, will resistthe baking temperature necessary to set the helix and any highertemperature to which the neck is subjected during processing subsequentto application thereof; and Which will not deteriorate as the tube ages.

A preferred mode of applying the helix is indicated in FIG. 6 wherein areservoir 61 holds a quantity of suitable paint 62. A wick 64 hearing onan applicator wheel 65 transfers the paint to the periphery thereof. Afixed rod 67 supports the wheel 65 for free rotation and the reservoiris freely suspended on the rod 67. The periphery of the wheel will besuch as to be frictionally rotated by contact with the neck, this latterbeing rotated while supported in a cradle in any well-known manner.Means (not shown) such as a lead screw and a nut secured to the rod 67,are provided to advance the wheel, reservoir, etc. at a constant ratecorresponding to the pitch of the helix. Thus, as the neck and wheelrotate in synchronism, the paint is fed from the reservoir onto the neckto produce the helical element. A gentle flow of air into the neck whileapplying the paint will avoid condensation of volatile components of thepaint. Following completion of the element, the applicator means isretracted radially inwardly of the tube neck and withdrawn, whereafterthe still moist element may be baked to a solid consistency.

As alluded to hereinbefore, in the present case and in any of the otherembodiments, the element may bE applied to a separate neck section,baked and the section attached to the funnel of the tube or the same maybe applied where the neck is already a part of the tube.

If the power supply serving the electron gun is of such character thatconnection of the low potential end of the element to G3 iscontra-indicated, as for example, by reason of unavoidable variation incross-section in the helix, it is within the scope of the invention toextend the helix rearwardly and to connect that termination to a pointof lower potential or even to ground.

As one specific embodiment of the invention as applied to the 21CYP22tube, a total length of helix of 2%" is selected and calculated toprovide a drop of say 20,000 volts, i.e., 24,500 volts at G4 and 4,500volts at G3. The drop per inch will then be volts per inch,approximately. Assuming a practical width of each turn of the helix as0.010" spaced apart 0.010", or a pitch of 0.020", the turns per inchwill be 50, or a drop per turn of volts. Sparking between turns at thisvoltage in vacuo is not likely but, as a precaution, insulation may beadded in the form of say ferric oxide (Fe O in a suitable vehicle.

It will be evident that the cross-sectional area of the element and thecoeificient of resistivity will be so related to the voltage drop over apredetermined total length of helix as to provide a safe value ofcurrent. Assuming a value of current of 5 a. and a drop from end to endof 20,000 volts, the total resistance will be 4000' megohms. Inasmuch asthe width of the helix has been assumed to be 0.010" and its thicknesswill, as a painted helix, be of an even lesser order, its powerdissipation will be low.

It is within contemplation to provide the element as a helix wound ofsuitable wire and to cement the same on the inner surface of the neck.In such case the relatively flimsy helix may be supported on a temporarycage pending setting of the adhesive. Such cage could be of acollapsible nature in order that, when the adhesive has set, the cagecould be collapsed radially for removal. Obviously, the cement will beof a composition which will not give rise to deleterious gases when thetube is subjected to later steps at high temperatures or, even if thisshould occur, any gas which may be generated will be substantiallycompletely removed upon evacuation of the tube, and any residue capturedby the getter.

Where, herein, reference is made to a diiference of potential betweenpoints of high and low potential the same is intended to have a purelyrelative meaning depending upon the structure which, relative to theenvelope, and in the absence of the invention improvement, may give riseto arcing. Depending upon the use to which the tube is put, suchdifference in potential may range from approximately 10,000 volts to50,000 volts.

While we have shown particular embodiments of our invention, it will beunderstood, of course, that we do not wish to be limited thereto sincemany modifications may be made and we, therefore, contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of the invention.

We claim:

1. In combination with an evacuated tube including an envelope having aneck portion and an electron gun positioned in the neck portion toprovide a beam of electrons for impingement on a target forming part ofthe tube, the gun comprising a plurality of electrodes, one of whichoperates at some lower potential, the neck portion including an interiorsurface portion adjacent the electrodes, the spacing between theelectrodes and the surface portion and at least one of the potentialsboth being such that incipient arc discharge between a first point ofone electrode and a second point of another electrode may occur, theimprovement comprising means to neutralize arcing including: anelectrically conductive film having a relatively high resistance appliedto the surface portion of the neck substantially adjacent the electrodeof higher potential, a conductive coating having a resistance lower thansaid film, said coating applied to the surface portion of the necksubstantially adjacent the electrode of lower potential, said film andsaid coating contacting each other and being substantially coextensivewith the electron gun, said film being electrically connected to thehigher potential electrode and said coating being electrically connectedto the lower potential electrode, whereby the voltage drop between saidfilm and coating and the electron gun is essentially zero.

2. The combination in accordance with claim 1 wherein the resistance ofsaid film in relation to its thickness and the length between its endsis such that current flow is a minimum consistent with a controlledpotential distribution along its length.

3. The combination in accordance with claim 1 wherein the difierence inpotential between the higher and lower potentials is in the range offrom 10,000 to 50,000 volts.

4. The combination in accordance with claim 3 wherein the film includesiron oxide and manganese oxide in such proportions as to provide aresistance per square of from 10 to 10 ohms, and said coating includesiron oxide having a resistance substantially less than said film.

5. The combination in accordance with claim 1 further characterized byan electrically conductive sleeve shielding the electrode operating at alower potential and being intermediate said coating and the electrode,said sleeve being electrically connected to said coating.

References Cited DAVID J. GALVIN,

JAMES W. LAWRENCE, Examiner.

V. LAFRANCHI, Assistant Examiner.

Primary Examiner.

1. IN COMBINATION WITH AN EVACUATED TUBE INCLUDING AN ENVELOPE HAVING ANECK PORTION AND AN ELECTRON GUN POSITIONED IN THE NECK PORTION TOPROVIDE A BEAM OF ELECTRONS FOR IMPINGEMENT ON A TARGET FORMING PART OFTHE TUBE, THE GUN COMPRISING A PLURALITY OF ELECTRODES, ONE OF WHICHOPERATES AT SOME LOWER POTENTIAL, THE NECK PORTION INCLUDING AN INTERIORSURFACE PORTION ADJACENT THE ELECTRODES, THE SPACING BETWEEN THEELECTRODES AND THE SURFACE PORTION AND AT LEAST ONE OF THE POTENTIALSBOTH BEING SUCH THAT INCIPIENT ARC DISCHARGE BETWEEN A FIRST POINT OFONE ELECTRODE AND A SECOND POINT OF ANOTHER ELECTRODE MAY OCCUR, THEIMPROVEMENT COMPRISING MEANS TO NEUTRALIZE ARCING INCLUDING: ANELECTRICALLY CONDUCTIVE FILM HAVING A RELATIVELY HIGH RESISTANCE APPLIEDTO THE SURFACE PORTION OF THE NECK SUBSTANTIALLY ADJACENT THE ELECTRODEOF HIGHER POTENTIAL, A CONDUCTIVE COATING HAVING A RESISTANCE LOWER THANSAID FILM, SAID COATING APPLIED TO THE SURFACE PORTION OF THE NECKSUBSTANTIALLY ADJACENT THE ELECTRODE OF LOWER POTENTIAL, SAID FILM ANDSAID COATING CONTACTING EACH OTHER AND BEING SUBSTANTIALLY COEXTENSIVEWITH THE ELECTRON GUN, SAID FILM BEING ELECTRICALLY CONNECTED TO THEHIGHER POTENITAL ELECTRODE AND SAID COATING BEING ELECTRICALLY CONNECTEDTO THE LOWER POTENTIAL ELECTRODE, WHEREBY THE VOLTAGE DROP BETWEEN SAIDFILM AND COATING AND THE ELECTRON GUN IS ESSENTIALLY ZERO.